Rechargeable sodium alloy anode

ABSTRACT

This invention relates to a novel anode for use in batteries, and to batteries containing the anode, which anode comprises an alloy sodium.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to rechargeable electrochemical cells. Moreparticularly, this invention relates to such cells in which the anodecomprises an alloy of sodium and one or more other metallic andnon-metallic elements.

(2) Prior Art

Lithium and lithium alloys have been suggested for use as the negativeelectrode of electrochemical cells. For example, U.S. Pat. No. 4,002,492discloses elecrochemical cells having an anode consisting essentially oflithium aluminum alloys that contain lithium in amounts between about63% and 92% and the balance essentially aluminum. Anodes composed oflithium and aluminum are also disclosed in Rao, et al., J. Electrochem.Soc. 124, 1490 (1977), and Bensenhard, J. Electroanal. Chem., 94, 77(1978). European Pat. No. 0070107; Murphy et al., J. Electrochem. Soc.,126, 349 (1979) and Murphy et al., Mat. Res. Bull., 13, 1395 (1978)disclose batteries based on lithium and sodium intercalation in layereddichalcogenides.

In J. O. Besenhard and G. Eichinger, J. Electroanal. Chem., 78, I(1976), at pages 8 and 9 describe the difficulty of redepositing sodiummetal at room temperature from non-aqueous solutions.

SUMMARY OF THE INVENTION

The present invention provides an improved electrochemical cell whichcomprises an anode, having an anode active material, a cathode havingcathode active material and a non-aqueous electrolyte having an ionicsalt of the anode active material dissolved therein, the improvementcomprising an anode which consists essentially of an alloy of sodium.Several advantages flow from this invention. For example, use of thesodium alloy allows the use of sodium (which is inexpensive andabundant) as the anode active material in a secondary battery eventhough sodium is normally difficult to plate and strip in non-aqueouselectrolytes. The commonly observed failure mode of the secondarybatteries which arises from the dendritic growth of the anode whichshorts the cell when plating the active anode material back to anode isavoided by alloy formation. The lower activities of the Na alloy alsoprovide greater stabilities than that of pure Na and Li, and Li alloysin the electrolyte media which greatly improves its rechargeability.Moreover, sodium forms intermetallic compounds or alloys with highsodium content, as for example, Na₅ Pb₂, Na₁₅ Pb₄, Na₅ Sn₂, and Na₁₅ Sn₄which provides for higher discharge capacities.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As an essential component, the electrochemical cell of this inventionincludes an anode consisting essentially of an alloy of sodium. The typeof sodium alloy employed is not critical and can vary widely. Suchalloys can be binary, ternary or can contain more than three components.The other component or components can be metallic or non-metallic.Illustrative of alloys which are useful in the conduct of this inventionare binary sodium alloys such as sodium alloys of tin, lead, silicon,antimony, bismuth, tellurium, thallium, selenium, gold, cadmium,arsenic, mercury, cesium, gallium, and the like. Also illustrative ofuseful alloys are ternary sodium alloys such as sodium alloys of tin andlead, antimony and lead, selenium and tellurium, and the like. Usefulalloys include quaternary alloys such as sodium alloys of lead, tin andbismuth.

Preferred for use in the practice of this invention are sodium alloys inwhich the other component or components are metals. Particularlypreferred for use in the practice of this invention are alloys of sodiumand one or more metals selected from the group consisting of tin, lead,antimony, bismuth, selenium, tellerium, mercury and cadmium. Amongstthese particularly preferred embodiments, most preferred are ternary orbinary alloys of sodium, and tin, lead, bismuth and/or antimony.

The mole ratios of the components of the alloy can vary widely,depending on permissible ratios based on allowed interactions betweenthe components and the desired capacity of the anode. In general, sincesodium is the electroactive material in the anode, the greater the molepercent of sodium in the anode, the greater the capacity of the anode;and conversely, the lower the mole ratio of sodium in the anode, thelower the capacity. In general, since higher capacities are desirable,higher amounts of sodium in the alloy are desirable. Sodium as comparedto lithium is readily adaptable to providing such high capacity anodesbecause it can form intermetallic compounds or alloys such as Na₅ Pb₂,Na₁₅ Pb₄, Na₅ Sn₂, and Na₁₅ Sn₄, which have higher sodium content. Inthe preferred embodiments of the invention, the mole ratio of sodium toother components in the alloy is about equal to or greater than about0.5 to 1. In these preferred embodiments, the upper amount of sodium inthe alloy is the greatest amount of sodium which can be alloyed with theother component or components before pure metallic, un-alloyed sodium isformed. In the particularly preferred embodiments of the invention, themole ratio of sodium to the other components in the alloy will usuallyvary from about 1 to about 1, to about 5 to about 1, and in the mostpreferred embodiments will vary from about 4 to about 1, to about 1 toabout 1.

The method of manufacturing the alloy is not critical and can varywidely. Conventional alloying procedures are readily adaptable for usein the practice of this invention. For example, such alloys can beformed electrochemically by plating sodium onto a substrate of the othercomponents as described in more detail in N. N. Tomashova, I. G.Kieseleva and B. N. Kabanov, Elektrokhimiya, vol 8, p112 (1972) which isincorporated herein by reference. Sodium alloys can also be preparedmetallurgically by melting appropriate amounts of sodium and the othercomponents in an inert atmosphere as described in more detail in R.Kremann and P. V. Reininghaus, Z. Metallkunde, vol 12, p273 (1920) whichis hereby incorporated by reference.

The anode of this invention may also include other optional materials.For example, in the preferred embodiments of invention, the anodeincludes a one or more polymeric binders. In these preferred embodimentsof the invention, the alloy is generally in particulate form, bondedtogether, and maintained in a composite structure by the binder. The useof the polymeric binders with the alloy in particulate form, providesfor a large surface area for the sodium alloy to contact with theelectrolyte media when the anode is placed in the battery electrolytesolution. Polymeric binders which are useful in the practice of thisinvention are those which form porous substrates to allow for contactbetween the particulate alloy and the electrolyte, and which hold thealloy in the composite during the charging and discharging of the cell.These functions can be provided by a single binder or a mixture ofbinders can be used each of which possess one or more of the desirablecharacteristics. Illustrative of such binders are polyphenylene, andother conjugated backbone polymers such as polythiophene, polyacetyleneand the like, and nonconjugated backbone polymers, as for examplepolyacrylates, polymethacrylates, polyvinyls polyethylene andpolypropylene. An especially useful polymer binder is polyphenylenebecause it swells and also becomes conductive when doped with sodiumcations, in combination with polyethylene or polypropylene. However,other materials can be used as binders, provided they are porous toallow for contact between the electrolyte and the alloy and allow thealloy maintains a good electrical continuity within the anode structure,and they maintain the alloy in the composite during the charging anddischarging of the cell. When swellable and dopable polymers such aspolyphenylene (PPP) are used, alloys having higher sodium content areemployed due to the fact that such polymers will absorb sodium from thealloy. Higher sodium content may not be needed when the swellablepolymer does not take sodium from the alloy.

The amount of binder used in the preferred embodiments of the inventionis not critical and can vary widely. Usually the amount of binder is notgreater than about 40 weight percent based on the total weight percentbased on the total weight of alloy and binder, and preferably from about10 to about 30 weight percent on the aforementioned basis.

The organic solvents which may be included in the electrolyte of thebatteries of the present invention may vary widely and can be organicsolvents normally used in batteries. Preferably, these solvents shouldbe electrochemically inert to oxidation and reduction during use whilesimultaneously being capable of dissolving the desired sodium metal saltand providing ionic conductiviities equal to or in excess of 10⁻⁴ S/cm.Examples of such useful organic solvents include propylene carbonate,ethylene carbonate, sulfolane, methyl sulfolane, dimethyl sulfolane,3-methyl-2-oxazolidone, alkane sultones, e.g., propane soltone; butanesultone (the use of soltones as electrolyte compositions is the subjectof a related, commonly-assigned U.S. patent application Ser. No.556,717, and the use of sultones for coatings on polymer anodes is thesubject of a related, commonly-assigned U.S. Pat. No. 4,472,489),dimethyl sulfoxide (DMSO), dimethyl sulfite, tetrahydrofuran (THF),2-methyltetrahydrofuran (2-MTHF), dioxane, dioxolane,1,2-dimethoxyethane (DME), dimethoxymethane, diglymes, glymes, anisole,nitriles, (e.g., proprionitrile, butyronitrile, acetonitrile,benzonitrile), dichloromethane, tetraethylsulfamide, aromatichydrocarbons, e.g., toluene, benzene, organo phosphorus compounds, e.g.,hexamehtylene and trimethyl phosphate. Mixtures of such availableorganic solvents may also be used, such as mixtures of propylenecarbonate and dimethoxythane.

The organic solvents chosen for use in any particular situation will, ofcourse, depend upon many factors such as the precise electrolytecomposition used and the voltage range desired, as well as the choice ofcathode and other components of the battery used. In the preferredembodiments of the invention ether-type solvents such astetrahydrofuran, dimethoxyethane, diglyme, 2-methyltetrahydrofuran andmixtures thereof are employed.

Salts for use in the electrolyte of the battery of this invention are ofthe formula:

    NaA

wherein:

Na is sodium; and

A is a species which is anionic in the electrolyte and stable underoperational conditions. Suitable anionic species include I⁻, Br⁻, Cl⁻,PF₆ ⁻, AsF₆ ⁻, SO₃ CF₃ ⁻, BF₄ ⁻, BCl₄ ⁻, AlCl₄ ⁻, alkylborates, such asB(CH₃)₄, arylborates, such as B(C₆ H₅)₄ ⁻ (benzoate), CH₃ C₆ H₄ SO₃ ⁻(tosylate), SiF₆ ⁻, HSO₄ ⁻ and the like. Preferred anions arealkylborates, arylborates, or alkylarylborates, PF₆ ⁻, and particularlypreferred anions are alkylborates, arylborates, ClO₄ ⁻, alkylarylboratesand PF₆ ⁻.

Molten salts may also be employed as the electrolyte of the battery ofthe invention. Room temperature molten salts suitable for use inbatteries of this invention include alkali metalhalide-trialkylimidazolium chloroaluminate, alkali metalhalide-dialkylimidazolium chloroaluminate, and alkali metal halide alkylpyridinium chloroaluminate. Since in many cases the polymers, alloys andother ion inserting materials of this invention are stable at elevatedtemperature, intermediate temperature molten salts (M.P. <200° C.) suchas MaAlCl₄ or KAlCl₄, are also suitable for use.

Cathodes for use in the practice of this invention are not critical andcan be varied widely. Suitable cathodes include a material selected fromthe group consisting of graphite, intercalation compounds of graphite,high surface area carbons (>200 M² /g), transition-metal chalcogenides,metal oxides, and conjugated backbone polymers which are capable ofbeing oxidized (acceptor-doped). Transition-metal chalcogenides, metaloxides and conjugated backbone polymers are preferred cathode materials.

The transition-metal chalcogenides, suitable as cathode materials usefulin this invention, can contain inserted alkali metals and include thetransition-metal dichalcogenides such as TiS₂ and, among others, thoselisted on page 392 of "Lithium Batteries" edited by J. P. Gabano(Academic Press, 1983) and in K. M. Abraham, Solid State Ionics, Vol. 7,pp. 199-212 (1982) both incorporated herein by reference). These include(with aproximate open circuit potentials measured in various organicelectrolytes when fully charged or devoid of inserted cations.

    ______________________________________                                        Na.sub.x TiS.sub.2 2.1 V versus Na/Na.sup.+                                   Na.sub.x NbS.sub.2 Cl.sub.2                                                                      2.2 V versus Na/Na.sup.+                                   Li.sub.x MoX.sub.2 2.3 V versus Li/LI.sup.+                                   Li.sub.x Fe.sub.0.25 V0.75S.sub.2                                                                2.4 V versus Li/Li.sup.+                                   Li.sub.x TiS.sub.2 2.5 V versus Li/Li.sup.+                                   Li.sub.x MoS.sub.3 (amorphous)                                                                   2.5 V versus Li/Li.sup.+                                   Na.sub.x WO.sub.3-y                                                                              2.6 V versus Na/Na.sup.+                                   NaMoS.sub.3 (amorphous)                                                                          2.7 V versus Na/Na.sup.+                                   Na.sub.x TaS.sub.2 2.7 V versus Na/Na.sup.+                                   Li.sub.x MoO.sub.3 2.8 V versus Li/Li.sup.+                                   Li.sub.x V.sub.6 O.sub.13                                                                        2.9 V versus Li/Li.sup.+                                   Li.sub.x Cr.sub.0.5 V.sub.0.5 S.sub.2                                                            2.9 V versus Li/Li.sup.+                                   Li.sub.x W.sub.0.2 C.sub.2.8 O.sub.7                                                             3.0 V versus Li/Li.sup.+                                   Li.sub.x Cr.sub.3 O.sub.8                                                                        3.6 V versus Li/Li.sup.+                                   Na.sub.x CoO.sub.2 3.7 V versus Na/Na.sup.+                                   Li.sub.x CoO.sub.2 4.7 V versus Li/Li.sup.+                                   ______________________________________                                    

Suitable polymer cathodes include oxidized polyacetylene,poly(p-phenylene), polyacenes, poly(phenylene vinylene), polyazulene,polynaphthalene, poly(phenylene sulfide), poly(phenylene oxide),polyphenothiazine, polyaniline, polypyrrole, polythiophene,polythianthrene and substituted verisons of the above. Such polymers maybe coated by reaction, when oxidized, with pyrroles, thiophenes,azulenes, oxiranes, anilines or furans, as described in commonlyassigned U.S. Pat. No. 4,472,987, the disclosure of which isincorporated herein by reference.

The secondary battery of this invention can be charged and discharged inaccordance with the procedure described in U.S. Pat. No. 4,602,492. Suchprocedures are well known to those of skill in the art and will not bedescribed herein in any great detail.

The following specific examples are presented to more particularlyillustrate the invention and are not to be construed as limitationstherein.

EXAMPLE I

To prepare Na-Pb alloy metallurgically, proper amounts of Na and Pb wereplaced in a stainless steel container and heated at a temperature about510±10° C. in inert atmosphere or under vacuum for a few hours and thenquenched to room temperatures. the alloy was then pulverized before use.Na--Pb alloy of the following composition NaPb₃, Na₅ Pb₂ and Na₁₅ Pb₄were prepared using the method described as above. Their electrochemicalpotentials relative to Na were measured using electrometer in theappropriate electrolyte solutions containing sodium ions and listed inTable 1 as follows:

                  TABLE 1                                                         ______________________________________                                        Composition  Potential (vs Na/Na+), V                                         ______________________________________                                        NaPb.sub.3   0.51                                                             NaPb         0.37                                                             Na.sub.5 Pb.sub.2                                                                          0.183                                                            Na.sub.15 Pb.sub.4                                                                         0.12                                                             ______________________________________                                    

EXAMPLE II

Metallurgically formed Na--Pb alloy powder of the composition Na₁₅ Pb₄(32 mg) was pressed onto the expanded nickel metal and used as anelectrode. The potential of this electrode was measured while stripping83% of the Na in the Na₁₅ Pb₄ alloy away from the electrode. Thepotential vs the amount of sodium stripped from the electrode wasplotted. Several potential plateaus were observed from the plot. Thisindicated that Na₁₅ Pb₄ and other binary alloys formed during thestripping procedure corresponded well with the potentials measure inExample 1 and with their respective compositions. Less than 100% Nastripping from the alloy was due to the loss alloy resulted from thevolume change.

EXAMPLE III

Sodium alloy can also be formed elecrochemically by plating Na onto Pbfoil. By plating Na on Pb foil at a current density of 50 microamp/cm²,Na--Pb alloy of various compositions were shown as several potentialplateaus. The potential of these plateaus: 0.5, 0.34, 0.17 and 0.12 Vcorresponded well with the values measured in Example 1.

EXAMPLE IV

Na ternary alloy containing Pb and Sn was prepared by heating the properamounts of Na, Pb and Sn in a stainless steel crucible in an inertatmosphere or under vacuum at a temperature about 510±10° C. for fewhours and then quenching to room temperatures. The resulting ternaryalloy had the following composition NaPb₀.26 Sn₀.74 and a potential of0.25 vs Na/Na⁺.

EXAMPLE V

A Na alloy electrode was prepared by mixing the NaPb₀.26 Sn₀.74 alloy ofExample IV in powdered form with 10 weight percent of polyphenylene(PPP) and 10 weight percent of polypropylene binder. The mixture wasthen pressed onto expanded nickel metal and heated at 160° C. for 10 to20 minutes under vacuum or in an inert atmosphere.

A cell was constructed which consisted of the anode (68 mg on an area of0.6×2.4 cm²) whose preparation is described above and a Na_(x) CoO₂cathode was assembled using glass filter paper to separate the twoelelctrodes. The electrolyte solution used was 1.25M NaPF₆ in DME.Before cycling, 3.77 mAh capacity was added to the anode. This amount ofcharge was obtained by charging the Na_(x) CoO₂ cathode and platingsodium from the auxilliary sodium electrode. The final potential of thiselectrode was about 0.10 V.

The cell was then cycled at a rate of 0.5 mA/cm² in the voltage range of3.2 to 2.2 V (Na_(x) CoO₂ vs alloy anode) for 22 cycles. The cell wasfurther cycled at a rate of 1.0 mA/cm² in the same voltage range fromthe 23rd to the 87th cycle. The potential profile of the anode (vsNa/Na+) vs capacity at selected cycles was plotted. The plot indicatedthat the discharge capacity of the cell decreased from 4.39 mAh at thefirst cycle to 3.59 mAh at the 21st cycle, 3.35 mAh at the 24th cycle,3.07 mAh at the 50th cycle and 2.84 mAh at the 87th cycle. The coulombicefficiency of each cycle was varied from 97 to 99%.

What is claimed is:
 1. A secondary battery comprising:(a) an anode whichcomprises an alloy of sodium and one or more metals selected from thegroup consisting of tin, lead, antimony, bismuth, selenium andtellerium, (b) an electrolyte comprising one or more organic solventsand one or more sodium salts dissolved therein forming dissolved sodiumcations in solution; and (c) a cathode; said sodium cations from saidelectrolyte alloying with said one or more metals of said alloy in saidanode during the charging of said battery and sodium in said alloydissolving in said electrolyte during the discharging of said battery.2. A battery according to claim 1 wherein said alloys and selected fromthe group consisting of ternary or binary alloys of sodium and one ortwo metals selected from the group consisting of tin, lead, bismuth, andantimony.
 3. A battery according to claim 2 wherein said alloy is abinary alloy of sodium and lead, or sodium and tin, or a ternary alloyof sodium, tin and lead.
 4. A secondary battery comprising:(a) an anodewhich comprises a ternery alloy of sodium and one or more other metallicor non-metallic materials; (b) an electrolyte comprising one or moreorganic solvents and one or more sodium salts dissolved therein formingdissolved sodium cations in solution; and (c) a cathode; said sodiumcations from said electrolyte alloying with said materials of said alloyin said anode during the charging of said battery and sodium in saidalloy dissolving in said electrolyte during the discharging of saidbattery.
 5. A battery according to claim 4 wherein said alloy is aternary alloy of sodium, tin and lead.
 6. A secondary batterycomprising:(a) an anode which comprises an alloy of sodium and one ormore other metallic or non-metallic materials wherein said alloycontains the maximum amount of sodium which can be alloyed with saidother components before pure metallic unalloyed sodium is formed; (b) ananode which comprises a ternery alloy of sodium and one or more othermetallic or non-metallic materials; (c) an electrolyte comprising one ormore organic solvents and one or more sodium salts dissolved thereinforming dissolved sodium cations in solution; and (d) a cathode; saidsodium cations from said electrolyte alloying with said materials orcomponents of said alloy in said anode during the charging of saidbattery and sodium in said alloy dissolving in said electrolyte duringthe discharging of said battery.
 7. A battery according to claim 6wherein the mole ratio of sodium to the other components is at leastabout 1 to
 1. 8. A battery according to claim 7 wherein said mole ratiois from about 1 to 1, to about 5 to
 1. 9. A battery according to claim 8wherein said mole ratio is from about 1 to 1 to about 4.5 to
 1. 10. Asecondary battery comprising:(a) an anode which comprises an alloy ofsodium and one or more other metallic or non-metallic materials inparticulate form dispersed in one or more polymeric binders at least oneof said binders being swellable by a battery electrolyte allowingelectrical continuity between all or a part of the dispersed alloy,wherein said binders are polyphenylene in combination with polyethyleneor polypropylene; (b) an electrolyte comprising one or more organicsolvents and one or more sodium salts dissolved therein formingdissolved sodium cations in solution; and (c) a cathode; said sodiumcations from said electrolyte alloying with said materials of said alloyin said anode during the charging of said battery and sodium in saidalloy dissolving in said electrolyte during the discharging of saidbattery.